ADC's have quite rigid meta-builds; the majority opting for Infinity Edge, Beserker's Grieves, two Zeal items,a LW item, and a lifesteal item (not in that order). The majority of ADC's thus have three build options: which 2 zeal items and which lifesteal item are they going to take. Of course, some ADC's, especially MF and Jhin, opt for different itemisation, and some have other competitive options, like Vayne taking Rageblade, but outside of that even the Zeal choices are lacking. For the most part, only Ashe or Jinx take Hurricane, and the lifesteal options are nowadays mostly just Bloodthirster. The only major meta choice they have is whether or not they buy Guardian Angel. In very rare occasions, some ADC's have options for Essence Reaver.

Tanks have varied overall itemisation that allows them to be adaptable, but not "versatile". Once they're in game and it's coming to that they're building their tank items, it's more or less decided what they're going to be. To account for how well certain champions are doing, a lot of "options" are set in stone: if their crit ADC is popping the fuck off, and they have a Yasuo/ Tryndamere or something, you get Randuin's Omen. If their threats have a lot of healing, you get Thornmail. If their DoT mage is going to 1v5 your team, you get Adaptive Helm. The point is, while tank itemisation is nuanced, you're decisions amount to "will I make the correct choice or not?".

Fighters/ Bruisers/ Juggernaughts buy Trinity Force or The Black Cleaver first and then tank. Deciding between those two items is the only choice they have and is mostly already decided upon. That's the meta. Maybe they buy a Sterak's Gauge as well. To be fair, Riven, Yasuo, Udyr, and Jax have decent options. The majority do not.

Supports have a lot of itemisation choices that are varied and impactful. I honestly believe that Support is genuinely the best fleshed out class/ role in the game because of this, and for me is a major part of why I love the role so much. A lot of supports have real choices between the starting gold items, how much enchanter/ AP/ tank items they buy, and with Transcendence have even more build options (since every traditional support item had 10% CDR, excluding Ruby/ Sightstone). Although I'm hesitant to see it, the removal of Sightstone as an item you buy will also increase the diversity supports have.

I'd like to note again that this is all meta itemisation. In practice, you can find success with ADC's building Black Cleaver or Wit's End, and fighters building Death's Dance, for instance.

Mages have a lot of options. Your starting items vary between The Dark Seal, Doran's Ring, Corrupting Potion, or Spellthief's Edge, because mage supports are a thing. Mages usually buy a mana-AP item first and so have options between Morellonomicon (which being extremely cost effective and basically the AP-Trinity Force of stats for mages makes it the most commonly bought item), Rod of Ages, Hextech GLP-800, or Archangels staff.

After that you can choose between raw AP - i.e. Mejai's Soulstealer, Rabadon's Deathcap, or Void Staff. Or, you can from a burst option - Lich Bane (which doubles as a mana option), Luden's Echo, or Hextech Gunblade. Maybe you want more survivability but not outright tankiness? Zhonya's Hourglass and/ or Banshee's Veil have you covered! Or maybe you'd prefer to be beefier while also taking down enemy tanky champions, in which case Rylai's/ Liandry's are your go-to options.

TL;DR: Actually ADC/ Tank/ Fighter meta itemisation sucks while mage itemisation is the second best in the game, behind support. Despite the meta you can still find success with personal builds.